Color Atlas of Hematology - Practical Microscopic and Clinical ...

158Erythrocyte and Thrombocyte AbnormalitiesHematological Diagnosis of MalariaVarious parasites may be found in the blood stream, e.g., trypanosomesand filariae. Among the parasitic diseases, probably only malaria is ofpractical diagnostic relevance in the northern hemisphere, while at thesame time malarial involvement of erythrocytes may confuse the interpretationof erythrocyte morphology. For these reasons, a knowledge ofthe principal different morphological forms of malarial plasmodia isadvisable.Recurrent fever and influenza-like symptoms after a stay in tropical regionssuggest malaria. The diagnosis may be confirmed from normalblood smears or thick smears; in the latter the erythrocytes have beenhemolyzed and the pathogens exposed. Depending on the stage in the lifecycle of the plasmodia, a variety of morphologically completely differentforms may be found in the erythrocytes, sometimes even next to eachother. The different types of pathogens show subtle specific differencesthat, once the referring physician suspects malaria, are best left to thespecialist in tropical medicine, who will determine which of the followingis the causative organism: Plasmodium vivax (tertian malaria), Plasmodiumfalciparum (falciparum or malignant tertian malaria) and Plasmodiummalariae (quartan malaria) (Table 27, Fig. 56). Most cases ofmalaria are caused by P. vivax (42%) and P. falciparum (43%).The key morphological characteristics in all forms of malaria can besummarized as the following basic forms: The first developmental stage ofthe pathogen, generally found in quite large erythrocytes, appears as smallring-shaped bodies with a central vacuole, called trophozoites (or thesignet-ring stage) (Fig. 57). The point-like center is usually most noticeable,as it is reminiscent of a Howell–Jolly body, and only a very carefulsearch for the delicate ring form will supply the diagnosis. Occasionally,there are several signet-ring entities in one erythrocyte. All invaded cellsmay show reddish stippling, known as malarial stippling or Schüffner’sdots. The pathogens keep dividing, progressively filling the vacuoles, andthen develop into schizonts (Fig. 56), which soon fill the entire erythrocytewith an average of 10–15 nuclei. Some schizonts have brown-black pigmentinclusions. Finally the schizonts disintegrate, the erythrocyte ruptures,and the host runs a fever. The separate parts (merozoites) then start anew invasion of erythrocytes.In parallel with asexual reproduction, merozoites develop into gamonts,which always remain mononuclear and can completely fill theerythrocyte with their stippled cell bodies. The large, dark blue-stainedforms (macrogametes) are female cells (Fig. 57 d), the smaller, light bluestainedforms (microgametes) are male cells. Macrogametes usually predominate.They continue their development in the stomach of the infectedAnopheles mosquito.